Messed up model, quads?
While Lemon's answer about the shading sounds correct, and use of triangles is a subject that can be debated for hours, I thought I would add some suggestions for adjusting the topology you have.

I see two ways to quickly get rid of the triangle you have highlighted.


Add an edge loop &#9096; CtrlR that will terminate at the triangle. This adds a fourth vertex to the triangle turning it into a quad.





Split the mesh that connects the triangles on each side and add in an extra face loop which adds another triangle that you can merge into a quad. With the vertices selected press V to rip the selected vertices. You can also use &#9095; AltV to rip and fill in one step.